The present invention relates to a method and circuit arrangement for controlling an electrical load. The controlled electrical load particularly relates to an arrangement of light-emitting diodes, hereinafter called LEDs, wherein the electrical load is to be supplied with a nearly constant operating current.
Constant-current sources are primarily used to control an electrical load, especially a load of LEDs, LED chains and/or LED arrays. Diverse arrangements of LEDs are known; besides parallel and matrix connection of LEDs, the possibility of series connection of LEDs is known. In a series connection of LEDs, all LEDs are connected behind one another in a row; this connection is also called an LED chain. In order to operate LED chains, a constant current is generated and conducted through the LEDs. A voltage that corresponds to the sum of the forward voltages of all LEDs then appears across the LEDs.
To achieve a constant luminous efficacy, the current that flows through the LEDs must therefore be controlled in a temperature-dependent manner and nearly constant. This is achieved in a known manner using pulse width modulation of the supplied current. This modulated current is then used for brightness control of the LED chain by means of pulse width modulation.
The energy supply of the LEDS is accomplished, for example, by means of a step-up converter. Such a step-up converter is considered state of the art.
An LED cluster arrangement that is supplied with constant current is known from DE 20 2007 011 973 U1. The LED cluster arrangement is controlled by pulse width modulation.
DE 2006 059 355 A1 discloses a control device in a method for operating a series connection of light-emitting diodes.
DE 10 2005 058 484 A1 discloses a circuit arrangement and a method for operating at least one LED.
Problematic in the known state of the art, however, is the fact that there exist various control possibilities for electrical loads consisting of individual point loads or partial loads, especially LEDs. Problematic above all in this case is that the voltage varies as a function of the number of controlled and connected point loads or partial loads. In particular, if the partial loads are switched on or off, then the output capacitance, which is used for smoothing the voltage over the electrical load, should be discharged before rewiring and switching on the loads so that a discharge of the output capacitance will then not occur through the partial loads that are still closed and thereby possibly damage them.